In many industrial manufacturing processes it is desirable to measure the thickness of a web in the form of plate, sheet, strip or coil of material. Although the web may be stationary at the time of measurement, it is common to measure a web as it travels by a conveyer in a longitudinal direction in a path along or through some type of machinery. For example, in the rolling of steel to reduce the thickness of the steel to a nominal thickness, the web of steel is measured to assure that it is meeting particular thickness specifications.
Devices for making such measurements have been developed in the prior art using nuclear radiation as well as beams of light, including laser beams. Most of these measuring devices assume that the web is aligned along an assumed passline that is perpendicular to the light beam or other radiation. Consequently, any angular deviation of the actual passline of the web from the assumed passline results in an error in the measured thickness. There are prior art devices that are designed to stabilize and maintain the assumed passline angle. There are other devices that sense the passline angle of the web and correct the measurement for the detected passline angle by projecting a geometric image on the web and analyzing the shape and relative dimensions of the image to determine a passline angle. Other devices scan a line across the web and use timing to detect locations. Still others use light of differing wavelengths. Some devices require a complex arrays of light beams and, like those that project an image, require an equally complex algorithm for extracting the passline angle data. The more complicated that the hardware arrangements, the light arrays and the detection algorithms are, the more opportunities that exist for introducing errors into the measurements.
There is, therefore, a need for a thickness gauge that measures the actual passline angle of the web and uses that measurement to make measurement correction but that is not so complex as the prior art so that it is not only less costly but also eliminates many opportunities for errors.
There is also a need for such a thickness gauge that can make additional corrections for laser beam misalignment, temperature variations and for unknown errors that are an inherent part of any measurement apparatus.